Amplifying system



Nov. 1, 1932. A. D. SILVA AMPLIFYING SYSTEM Original Filed July 26, 1924 lfidenior MM iium W4 :4 $29M;

Patented Nov. 1, 1932 ALBERT n. srLvLor rnrLAnnLrnrn, rnNnsYLvANrA, Ass'rcmon ro n'rwarnn KEnr tin or "PENNSYLVANIA Original application filed July 26, 1924,

ldyinvention relates to thermionic amplifiers and their-associated circuitsutilizable for amplifyingfiuctuating currents or oscillations, and more particularly currents of the.

' structures within the related tube.

g.=jchar'acter .present in' radiogreceiving apparatus.

Th sapplication: 1s a d v1s1on from my application SerialNof 728,280, filedJuly 26,-

1924 1 c In accordance with my invention the stabilizing resistance or impedance of an amplie fier system, comprising a thermionic amplifierj or amplifier of the a-udion type having input andoutput circuits or systemsinclud sing loops or closed circuits tunable througln out-a range ofwave lengths, isutilized as described in my prior application aforesaid,in I

one; or .moreirelations as follows:

(a) In one or more stagesin eachof which the nput or; output transformer, or both, has

orliave substantial step-up ratio; and the inductance effective in theplatecr' anode Len-- cult s of relatively low magnitude.

(b) In one ,or nore stages in each of; which 251 the inputv or outputtranstormer, or both, has or haveloose coupling between the primary and secondary circuits. p P 7 (0) In a plurality of stages in each of whose input circuits or systems substantially goathe entire stabilizing resistance or impedance is connected in series between one terminal of the tunable loop and the gridywhile the other terminal ofthe tunable loop is connected to, or is at substantially the same. potential as aaith at of," the cathode or filament.

(d) ln'oneor more stages in each of which the stabi izing'efiect ofthe resistance or in1- pedance is complementalfor supplementalt'o the eiiectof additional means for reducing 40 or partially eliminating regeneration of feedback tending to contribute to production of oscillations. 1 I I ('6) With a detector of the thermionic or audion type in whose input circuit'or system 451s a tunableloop external-ate which and in Atari-Irvine sYs'rnM the stabilizing resistance or impedance.

.. MANUFACTURING ooiurnny, or'rnrnzanurrnra, rnunssnvnnm, A conr'o'an'rron Serial- No. 728,280; Dividedand this application filed January 19, i 192s. sna no. 333,683. 7

(f) With means for preventing substantial conduction between the grid and cathode I M invention resides in i the features of structure, a rangement and proportionmentl "for an illustration of one of the various forms it may take, reference 1s to be had to theiaccompanying drawing, whlch 1s a: diagram matic view of a radio receiving system.

The antenna D represents generically. an antenna, a loop antenna. or other structure ror ,eo' absorbing electro-radiant energy from the natural media, and represents also a circuit or path in or' operatively related to an artificial conductor system extending between transmltting and receiving stations for transmlttingslgnals, speech or controls-by carrier Between the antenna D and the earth or.

counter-capacity E is connected the primary of a IZtCllO. frequency. a1r-coretransformer T, Whose secondary is shunted by the.

to make it resonant with or sharply tunable variable tuning condenser C, which with said secondary comprises the tunable loop circuit per se whose electrical constants are such as to the received high frequency energy; The;

thermionic device V, as a vacuum tube of the audion type, comprises as usual the anode a, grid or control electrode 9 and filav ment or cathode f, andhas its input or grid c1rcu1t connected to the aforesaid tuned circuit. The anode and cathode at least are disposed 1n an envelope or tube evacuated to any suitable degree, and generally and preferably to that high degree effecting pure electron discharge.

7 It will be understood, however that my invention is applicable to any suitable degree of vacuum, and is a plicable also to those tubes containing gas. Thetube V operates as an amplifier, in

this example as a radio frequency amplifier, in whose anode circuit is connected the primary of a radio frequency air-core transformer T1 whose secondary constitutes an inductance shunted by the tuning condenser G1, which with its associated inductance constitutes a second resonant or tunable circuit or loop connected to or in the input circuit of a second amplifier V1, in whose anode circuit is the primary of a third radio frequency air-core transformer T2 Whose secondary is a tuning inductance shunted by the tuning capacity C2 and forming therewith a tunable or resonant circuit or loop connected to or in the grid circuit of thedetector tube V2, which may have characteristics similar to either of the tubes V, VI, or which may have different characteristics, as, for example, lower vacuum, suiting it to demodulating or detector purposes. In circuit with the grid, is the grid condenser K shunted by the grid leak resistance r. In the anode circuit of the tube V2, which is not tuned, or is at most broadly tuned, is included the instrument I, which may be a telephone, loud speaker or generically, any translating de vice, as, for example, the primary of an audio frequency transformer for coupling the anode circuit of the tube V2 with the grid circuit of an audio frequency amplifier, which may be the first of any suitable number'of stages of audio frequency amplification.

The cathodes f are heated by current from the battery A through the control rheostats H, by means of which a substantial amount of stabilization is effected or the tendency to productionof oscillations is reduced. This stabilization or reduction of tendency to production of oscillations increases with increase of resistance H in the filament circuit.

The anode circuits include the battery or source of current B.

Between the anode a and grid 9 of a thermionic device of the audion type exists a ca pacity which couples or assists in coupling the output and input circuits to each other. This coupling capacity, indicated in dotted lines at C8, lends itself in effecting what may be a regenerative coupling between output and input circuits which in many cases causes the production locally of parasitic or undesired oscillations. The prevention or suppression of the regenerative coupling, and the prevention of the production of such oscillations are objects of my invention, which, in another aspect, relates to the reduction or. annulment of the effect of the intra-tube coupling capacity.

In accordance with my invention, I introduce intoone of the circuits, particularly the input or grid circuits or system, externally, however, to the tuned or resonant circuit or loop per se comprising the tuning capacity condenser C, C1 or C2, and either in the filament or grid lead, but preferably,

as indicated, in the grid lead, a resistance or V impedance R, which may be a non-inductive resistance, or an impedance comprising re sistance and some inductive reactance, though a strictly non-inductive or substantially non-inductive resistance is preferred.

By utilization of a resistance R of suitable magnitude, the regenerative action between output and input circuits of a tube is reduced or annulled, the production of undesired or parasitic oscillations is prevented, and in gen: 7

ance R- suitable for the purposes described should, from the aspect of its damping -ef fect upon the tunable or resonant circuit or loop per se, be as small as possible to preserve to as great an extent as possible the sharp tuning qualities of that circuit, and, on the other hand, should be of amagnitude sutli ciently great to reduce or eliminate the effect of regenerative action and productionof oscillations.

The magnitude of the resistance or impedance R will be greater as the amplification factor of the associated tube or tubes is greater and as the grid-to-anodecapacity, C3, is greater, and is greater as the reciprocal of the anode circuit resistance or impedance within the tube is greater, and particularly as the external impedance or resistance between the anode and cathode with respect to fluctuating or alternating current and as influenced to some extent by the anode-cathode capacity, is greater.

Tubes, even when of the same type or make vary among themselves as regards the magnitudes of their amplification factors grid+toanode capacities, and plate circuit impedances, particularly their anode-to-cathode impedances; and furthermore, the relations-of these factors vary with varying, conditions;-

under which the tubes are employed. Incon sequence, it is desirable that the magnitude of the resistance R be suiliciently high to suit a maximum valueof the product ofthe amplification factor, grid-to-anode capacityand reciprocal of the internal anode circuit or anode-to-cathode impedance;

I have found thata suitable magnitude of:

nssaeoi R=lam0P10- in which, m is the amplification factor,

the grid-to-plate capacity in micro-microfarads, k is a factor of the .order of 4 for two or more amplifier tubes in-cascade, and P the impedance of the plate circuit external to the tube.

Taking, forexample, a UV 201 A tube now upon the market, whose grid-filament capacity is of the order of 6 micro-microfarads, and for which on is 8, C 'is '6, and WithP 3000 ohms, R for each of the stages should be given a magnitude of the order of 550 to 650 ohms. For these conditions stated, a resistance R of this magnitude will prevent production of the undesirable oscillations and reduce or suppress the effect of inherent re generative action, and yet will not materially. affect the sharpness of tuning of the tunable or resonant clrcult or loop per secomprising any of the condensers C, C1 and G2 with its associated inductance, whlch is in the example illustrated, a secondary of a radio frequency transformer and of fixed magnitude. However, UV QOl-A tubes vary among themselves as to the aforesaid factors, and to take care of variations consisting in increases in the amplification factor and grid-to-anode capacity and increase in the impedance P, the resistance R is in practice preferably given a magnitude greater than its aforesaid value,

but not sufficiently greater seriously to affect,

the sharp tuning or resonance qualities of the tunable circuit or loop per se.

In case of employment of but a single radio frequency amplifier tube, as distinguished from the cascade or staged relation above referred to, the impedance or resistance R will have a magnitude preferably of the order of twice that given by the foregoing empirical formula and example. f

The use of the resistance R in the input circuit of a detector tube, as V2, and in other relations where currents of radio or audio frequency are to be amplified, is of advantage notwithstanding there may not be present sharp resonance or tuning, as in the case of the anode circuit of thedetector tube which is.

not sharply tuned but'is at best broadly tuned. The utilization of the resistance R, however, finds its principal application in the input circuits of tubes, such as V and'Vl, used simply as radio frequency ampliers.

It is preferred, as to each of the input circuits with which the resistance R is associated, that the grid 9 be negative, or at any rate not sufficiently positive to effect substantial conductivity between grid .and cathode f. In the example illustrated this is effected,

by connections of the grid circuits to those terminals. of cathodes 7 which are connected to the negative terminal of the battery.

This effects substantially pure capacity be tween the input electrodes 9 and 7; and avoids, substantialconduction between them which would produce damping, of the tunable cir suits, of a less desirable relation with respect to frequency than is ah orded byresistances E in series with'small substantially pure capaciti'es. I 1 I t is further preferable that for the wave lengths for which the receiving apparatus is to be utilised, or throughout a part or whole of a range of different wav lengths for which is to be utilized, the inductance associated with each of the condensers C, C1 and C2 and constituting the secondary of the transformers T, T1 and T2 shall be the predominating frequency-determining element of the tuned circuit or loop per se in the sense that such inductance is large compared with the associated tuning capacity, whereby the persistency of each tuned circuit is high or its'decrement is small, with the result that such damping effect as the resistance Rmay have upon the tuned circuit is suitably small;

For example, for a wave length range of the order of from 200 to 600 meters, the sec-- ondary of each of the transformers T, T1 and T2 may have an inductance of the order to .3 millihenry, and each of the associated tuning capacities C, Cl and C2 may have a magnitude varying from about .00003 micro-farad for 200 meters to about .00027 for 600 meters and voltage, and in the example given, the

inductance of the primary is of the order of .022 or .03 millihenry, the step-up of voltage is of the order of l to 10, the secondary voltage being of the order of ten times that of the primary. Furthermore the coupling between primary and secondary windings is loose, in the sense that it is substantially and materially less than unity or very close coupling. V

A fixed inductance, as a winding of acoupling transformer, with its variable condenser constitutes a closed or loop circuit tunable throughout a wave length range and comprised in an input or output system; and across the terminals of the inductance or the tuning condenser are connected the cathode and one of the other electrodes of a tube. The resistance R is external to the tuned loop. This particular position of the resistance, external to the tuned loop or circuit, in series with the small capacity between grid and cathode, in a path in shunt to the loop, is of importance and advantage, as distinguished from connection of the resistance in series within the tunable closed or loop circuit, or of resistance in shunt to the loop without the small capacity. By the external shunt connection of the resistance in series with small capacity it becomes markedly effective in reducing the tendency to increased. instability with increasing frequency, with the input and output systems tunable, as shown, by lined inductance and variable capacity. This is in marked contrast with the behavior of a fixed resistance in series in the grid circuit, external to the tube, but in series in the tunable loop or closed circuit, where the stabilizing or damping effect of the resistance is constant for all frequencies to which the loop or closed circuit is tuned. When the stabilizing resistance or impedance R is connected in series in the grid circuit, external to the tube, in a position where it is also external to the tunable closed circuit or loop and in series with tue small capacity formed by the grid and cathode, its efiect upon the closed circuit or loop for any frequency to which it is tuned is inversely proportional to the square of the magnitude of the capacity which tunes the loop or closed circuit at that frequency, or is proportional to the fourth power of the frequency to which the loop or closed circuit is tuned. Notwithstanding the resistance R is not varied or is of fixed magnitude, its effect varies automatically in proportion to the fourth power of the frequency when in the described position and in series with the small capacity formed by the grid and cathode. To procure the same effect by resistance in series in the tunable loop or closed circuit, requires an adjustment very rapidly increasing the magnitude of the resistance with increase in frequency.

The regenerative effect, or the negative resistance introduced by regenerative action into the tuned grid circuit, is proportional to the fourth power of the frequency. The effect of positive resistance R accordingly rapidly increases at the same rate of increase of the aforesaid negative resistance with increase of frequency to which the system is tuned, and resistance R, in the relation described, is well suited to oppose or compensate for regenerative effects. When the tuning capacity is large for the low frequency portion of the wave length range where the tendency to instability is small, the effect of the externally connected resistance R and small grid-cathode capacity is small, while when the tuning capacity is of low magnitude, for the high frequency portion of the wave length range, where the tendency to instability is great, the efiect of the externally connected resistance R- in series with small inter-electrode capacity is very. great. Throughout the wave length range to which the systemis tunable the re actance of the small capacity, in series with the stabilizing resistor R, preponderates over or is high relatively to the resistance of the resistor R.

By cooperation with the stabilizing resistance R, positioned and having the effect described, other features hereinbefore described are of advantage by way of reducing the stabilizing effects of other resistances inherent in the system and which are proportional to lower powers of the frequency than the aforesaid fourth power effect of the resistance R. A

Because of the loose coupling, and more particularly because of the substantial stepup ratio, of the transformer T, which couples the absorption structure or antenna system DE to the input circuit of the tube V, the damping effect of the absorption structure or antenna system upon the input circuit or tunable loop of the input system of the tube V, is materially reduced. In consequence, the damping produced by the resistor R is a greater proportion of the total damping effective in the loop tunable by condenser C.

Similarly, by reduction of the damping of the tube V or V upon the tunable loop of the input system of the tube to which its anode circuit is coupled, and by reduction of the damping effected by prevention of substantial conduction between grid and cathode, both about to be described, the damping produced by resistor R becomes a greater proportion of the total damping effective in the tunable loop to which it is related.

Because of the loose coupling, and particularly because of the high step-up ratio, of the transformer T or T the damping effect upon the tunable loop in the input circuit of the tube V or V due to theresistance of the tube V or V particularly within it between its anode a and cathode f, is greatly reduced, and the damping, produced by the resistor R becomes a greater proportion of the total damping effective in the tunable loop of the circuit or system with which resistor R is related. Such resistance transferred by the transformer T or T to the loop tunable by the condenser C or C is equivalent to a resistance in series within the loop tunable by C or C whose clamping effect is proportional to the square or second power of the frequency. By the loose coupling or substantial step-up ratio aforesaid, the damping effect proportional to the second power of the frequency is materially reduced, and in consequence the stabilizing effect of the resistance it plus that of the internal resistance of the preceding tube between its anode and cathoderemains more nearly proportional to the-fourth power coupling and high step-up ratio.

of the frequency, characteristic of the stabil izing effect of the resistance R alone.

Resistance within the tube, such as might be'due to conduction from the grid g tothe cathode f, is not, though external to the tuned loop of the input system, in series with V a small capacity, and in consequence its stabilizing effect is also the equivalent of that of a resistance in series in the tuned loop and whose effect varies with the square of the frequency. By reducing such inter-electrode conduction within the tube,.-thetotal of its effect plus that of the resistance; R remains more nearly proportional to the fourth power of the frequency, characteristic of the resistance R alone. In the'present case the inter-electrode conduction, as that'between the grid 9 and cathode f of a tube, is made inconsequential or negligible by the aforesaid negative bias of the grid 9 with respect tothe filament or cathode f. v 1

The regenerative action, through the anode-grid capacity to which the magnitude of resistance R is proportional, is reduced or limited by the low effective inductance in the anode circuit'of the tube, as effected by the low inductance of the primary winding, parti-cularly when in or of a transformer of loose The stabilizing efiect'of resistance R is supplemented byanother stabilizing effect procured by the resistance or rheostat H by which the cathode-heating current may he graduated or controlled, by inclusion in the filament circuit of suitable amount of the resistance H; the greater the amount of the resistance H in circuit with its filament f the smaller is the regenerative effect or feed-back actionyand the greater the stabilizing effect by filament current control. The low anode circuit inductance andcontrol of filament or cathode current individually and jointly complement or supplement the stabilizing effect of theresistance or impedance "R, in procuring a stable or non-oscillating:amplifier system.

Ina multistage system, such as hereinbe fore described, the filaments or cathodes are generallygrounded, and the rotors or adjustable elements of the tuning condensers are those plates or armatures of the tuning condensers which are connected to the oathodes or grounded. To maintain this arrangement substantially the entirestabilizing resistance R in each stage is disposed in the connection from the other armature or plate of the tuning condenser to the grid 9. lVith the entire or a substantialpart of the total resistance R of a stage connected between the lower terminal of the tuned-loop and the filament or cathode j", the lower plate or armature ofthetuningcondenser,generally the rotor as stated, will be at a potentialsubstantiallydifferent from that of the cathode, which is undesirable for the well-knownrea son that approachof the handy of the operator to the adjustable element or armature 0fv the tuning condenser has less effect when that plate or armature is atsubstantially the potential of thegrounded-cathode or filament than when it isat a substantially different potential. I I i I For brevity in the appended claimsthe term'resistance is used in a generic sense to include non-inductive resistance and resistance which maybe more or less inductive.

What I claim is:

1.- The combination with a thermionic'amplifier,'..of an input system'therefor tunable throughout a range of frequencies by fixed inductance and a variable condenser effectively in shunt therewith to form a tunable the tunable loop thereof and in series with i a small capacity, whereby the aforesaid 'ten dency of the amplifying system to instability is compensated for by said resistance whose stabilizing effect rapidly increases with increase of frequency.

' 2. The combination with a thermionic amplifier, of an input system therefor comprising a loop tunable throughout a range of frequencies, an output system comprising a loop tunable throughout said range of frequencies, a capacitative coupling between said output and input systems, the tendency of the amplifyingsystem tobecome unstable increasing'as said loops are tuned to higher and higher frequencies of said range, means for preventing substantial. conduction between the input electrodes of said amplifer, a fixed'resistance external to the tunable loop of said input system and in series with said input electrodes, whereby the stabilizing 'effect of said resistance rapidly increases with increase in frequency,- and supplemental means limiting the regenerative action be-- tween said output and input systems through said capacitative coupling.

3. The combination with a thermionic amplifier, of an input system therefor comprising aloop tunable throughout a range of frequencies, an output system comprising a loop tunable throughout said range of frequencies, a capacitative coupling between said output and input systems, the tendencyof the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher frequencies of av Hi) said range, the inductance effective in the said systems external to the tunable loop thereof and in series with a small capacity, whereby the aforesaid tendency of the amplifying system to instability is compensated for by said resistance Whose stabilizing effect rapidly increases with increase of frequency, and supplemental means for limiting the regenerative action between said output and input systems through said capacitative coupling.

' f. The, combination with a thermionic amplifier, of an input system therefor tunable throughout a. range of frequencies by fixed inductance and a variable condenser effectively in shunt therewith to form a tunable loop, an output system tunable throughout said range of frequencies by a fixed inductance vwith avariable condenser effectively in shunt therewith, a capacitative coupling between said output and input systems, the tendency of the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher fre quencies of said range, a resistance in said inputsystem external to the tunable loop thereof and in series with a small capacity, whereby the stabilizing effect of said resistance rapidly increases with increase in frequency, a system traversed by currentto be amplified, and a loose coupling between said last named system and said input system for reducing the damping effect thereof upon said input system, whereby the proportion of the total damping effect produced by said resistance is increased.

5. The combination with a thermionic amplifier, of an input system therefor comprising a loop tunable throughout a range of frequencies, an output system comprising a loop tunable throughout said range of fre quencies, a. capacitative coupling between said output and input systems, the tendency of the amplifying system to become unstable increasing as said input and output systems are "tuned to higher and higher frequencies of said range, a fixed resistance in said input system external to the tunable loop thereof and in series with a small capacity, whereby the stabilizing effect of said resistance rapidly increases with increase in frequency, a second thermionic device, and means for loosely coupling the output circuit of said device to said input system to reduce the damping effect of said device upon said input system, whereby the proportion of the total damping effect produced by said resistance is increased.

6. The combination with a th rmionic amplifier, of an input system therefor comprising a loop tunable throughout a range of fre quencies, an output system comprising a loop tunable throughout said range of frequeninput system to reduce the damping effect thereof upon said input system, whereby of the total damping effect the-proportion produced by said resistance is increased.

7. The combination with a thermionic amplifier, of an input system-therefor comprising a loop tunable throughouta range of frequencies, an output system comprising a loop tunable throughout said range of frequencies, thetendency of the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher frequencies of said range, resistance in said input system external to the tunable" loop thereof and in series with a small capacity, whereby the stabilizing effect of said resistance rapidly increaseswith increase in frequency, a system traversed" by current to be amplified, and means for increasing the proportion of the total damping effect produced by said resistance*comprising a magnetic coupling of substantial step-up ratio between said last named system and said'input system.

8. The combination with a thermionic amplifier, of an input system therefor comprising a loop tunable throughout a range of frequencies, an output system tunable throughout said-range of frequencies, the tendency of the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher frequencies of said range, a resistance in said input system external to the tunable loop thereof and in series with a small capacity, whereby the stabilizing effect of said resistance rapidly increases with increase in frequency, a second thermionic device, and means forincreasing the proportion of the total damping efiect produced by said resistance comprising a magnetic coupling of substantial step-up ratio between the anode circuit of said device and said input system.

9. The combination with a thermionic amplifier, of an input system therefor comprising a loop tunable throughout a range of frequencies, an output system tunable throughout said range of frequencies, in shunt therewith, the tendency of the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher frequencies of said range, a resistance in said input system external to the tunable loop thereof and in series with a small capacity, whereby the stabilizing effect of said resistance rapidly increases with increase infrequency, an'antenna system, and means for increasing the proportion of the total damping effect produced by said resistance comprising a magnetic coupling of substantial step-up ratio between saidantenna systeni and said input system.

10. The combination 'with thermionic amplifier, of an input system therefor comuprising a: loop tunable throughout a range of frequencies, an output system 'tunabl throughout saidrange of frequencies, a caacitative cou lin between said out ut and input systems, the tendency'of the amplifythe tunable loop thereof and in series with a small capacity,. the stabilizing effect of said resistance rapidly increasing with ining system to becomeunstable increasing as said inputand output systems are tuned to higherand higher frequencies of said range,

a resistance in said inputsystem external to crease of frequency, a system traversed by current to be amplified, a magnetic coupling of substantial step-up ratio between said last named system and said input system, a transformer of substantial step-up ratio Whose primary is in the anode circuit of said amply tier the inductance effective in the anode cir- 7 :cuit of said amplifier having relatively low magnitude, andsupplemental means limitingthe regenerative action between the out put and input circuits through'said capacitative coupling.

' a resistance in said input system external to 11. The combination with a thermionic am lifier of an in ut s stem therefor comprising a loop tunablethroughout a range "of frequencies, an output system tunable throughout said range of frequencies, a capacitative coupling between said output and input systems, the tendency of the amplifying system to become unstablei increasing as said input and output systems are tuned to higher and higher frequencies of said range,

the tunableloop thereof and in series with the input electrodes of said ampllfier, means 'reventin substantial conductionbetween said input electrodes of said amplifier, the stabilizing effect of said resistance rapidly increasing with increase of frequency, asysv tem traversed by current to be amplified, a

magnetic coupling of substantial step-upratiobetween said last named system and said input system, a transformer of substantial step-up ratio whose primary is in the anode circuit of said amplifienflthe inductance effective'in the anode circuit of said amplifier having relatively low magnitude, and supplemental means limiting the regenerative action between the output and input systems through said capacitative coupling.

12. The combination with a thermionic detector, of an inductive output system therefor, an input system therefor including a tunable loop comprisinginductance and a variable condenser effectively in shunt therewith, and a fixed resistance in saidinput system external to the tunable loop thereof and in series with a "small capacity for producing a stabilizingeifectwhich rapidly.

increases with increase in frequency.

in said input system external to the'tunable loop thereof inseries'with the input electrodes of said amplifier, and whose damping effect uponsaid tunable loop ra 1y increases with increase in frequency, a: system traversed by current to be amplified,

and means for increasing the proportion'of the total damping effect produced by said resistance comprising a loose coupling between said last named system and said input system, supplemented by means preventing substantial conduction within said'a-mplifier between its said input electrodes.

14'. The combination with a thermionic amplifier, of an input system therefor compris-;

ing a looptunable throughout a range'of frequencies, an output system tunable through out said range of frequencies, the tendency of the amplifying system to become unstable increasing as said input and output systems" are tuned to higher and higher frequencies of said range, a resistance in said: input system external to the tunable loop thereof in series with the input electrodes of said amplifier, and whose damping effect upon said" tunable loop rapidly increases with increase in frequency, system traversed' by current to be amplified, and means for increasing the proportion of the total damping effect produced by said resistance comprising a mag-"- netic coupling of substantialstep-up ratio between said last named system and said input system, supplemented by means preventing substantial conduction within said amplifier between its saidinput electrodes.

15. The combination with a thermionic amplifier, of an inputsystem therefor comprising a loop tunable throughout a range of frequencies, an output system comprising a loop tunable throughout said range of frequencies, the tendency of the amplifying system to become unstable increasing as said loops'are tuned to higher and higher frequencies of said range, a resistor of fixed resistance external to the tunable loop of one of said systems in series with a capacity, the

magnitude of whose reactanceat the frequencies ofrsaid range 'pr'e'ponderates over the magnitude of said resistance, whereby the. stabilizing effect of said resistor rapidly 1n-j tunable creases with increase in frequency, an associated system, and a coupling of substantial step-up ratio coupling said one of said systems to said associated system.

16. The combination with a thermionic amplifier, of an input circuit therefor comprising a loop tunable throughout a range of frequencies, an output circuit comprisi a said resistance and reactive element varying at a rate greater than the second power of the frequencies to which said input and out put circuits are tuned, an associated circuit, and a magnetic coupling of substantial transformation ratio between said one of said circuits and said associated circuit.

17. The combination with a thermionic amplifier, of an input system therefor comprising a loop tunable throughout range of frequencies, an output system tunable throughout said range of frequencies, stabilizing means external to the tunable loop of said input system comprising resistance in combination with a reactive element ofthe same kind as that which is variable to tune the loop of said input system, the stabilizing effect of said resistance and reactive element rapidly varying at a rate greater than the second power of the frequencies to which said input and output systems are tuned, a system traversed by current to be amplified, and means for increasing the proportion of the total damping effect produced by said stabilizing means comprising a magnetic coupling of substantial step-up ratio between said lastnamed system and said input system.

18. The combination with a thermionic amplifier, of an input circuit therefor comprising a loop tunable throughout a range of frequencies, an output circuit comprising a loop tunable throughout said range of frequencies, only the tuning elements of said input and output circuits being variable, the tendency of the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher frequencies of said range, stabilizing means external to the tunable loop of one of said circuits comprising resistance in combination with a reactive element of the same kind as that which tunes the loop of said one of said circuits, the effect ofsaid stabilizing means varying at a rate greater than the second power of the frequencies to which said input and output circuits are tuned, an associated circuit, and a magnetic coupling of substantial transformation ratio between said one of said circuits and said associated circuits.

19. The combination with a thermionic amplifier, of input and output systems therefor tunable throughout a range of frequen-,. cies, the tendency of the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher frequencies of said range, stabilizing means whose effect varies at .a rate greaten amplifier, of an input system therefor tunable...

throughout a range of frequencies byYfixed inductance and a variable condenser effectively in shunt therewith to form a tunable loop, an output system tunable throughout said range of frequencies by a fixed induc tance with a variable condenser effectively in shunt therewith, acapacitative coupling between said output and input systems, the tendency of the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher frequ-encies of said range, the inductance effective in the anode circuit of said amplifier having relatively'low magnitude, and a resistance in one of said systems external to the tunable loop thereof and in series with an inter-electrode capacity, whereby the aforesaid tendency of the amplifying system to instability is compensated for by said resistance Whose stabilizing effect varies substan-y tially as the fourth power of the frequency to which said input and output systems are tuned.

21. The combination with a thermionic amplifier, of an input system therefor comprising a loop tunable throughout a range of frequencies, an output system comprisin a loop tunable throughout said range of requencies, the tendency of the amplifying system to become unstable increasing as saidloops are tuned to higher and higher frequencies of said range, a fixed resistance external to the tunable loop of said input system in series with a small capacity, whereby the stabilizing effect of said resistance varies sub stantially as the fourth power of the frequencies to which said loops are tuned, and supplemental means limiting the regenerative action between said output and input systems through said capacitative coupling.

22. The combination with a thermionic amplifier, of input and output systems there for tunable throughout a range of frequencies, the tendency of the amplifying system to become unstable increasing as said input and output systems are tuned to higher and higher frequencies of said range, stabilizing means included in one of said systems whose efiect varies at a rate greater than the second power of the frequencies to which said input and output systems are tuned, and means for reducing stabilizing efiect introduced into said one of said systems by couplin to an adjacent system, which stabilizing e ect varies at a rate proportional to a power lower than the second power of the frequencies to which said input and output systems are tuned.

ALBERT D. SILVA. 

